Optical Bragg accelerators

Abstract

It is demonstrated that a Bragg waveguide consisting of a series of dielectric layers may form an excellent optical acceleration structure. Confinement of the accelerating fields is achieved, for both planar and cylindrical configurations by adjusting the first dielectric layer width. A typical structure made of silica and zirconia may support gradients of the order of 1 GV/m with an interaction impedance of a few hundreds of ohms and with an energy velocity of less than 0.5c. An interaction impedance of about 1000 Omega may be obtained by replacing the Zirconia with a (fictitious) material of epsilon=25. Special attention is paid to the wake field developing in such a structure. In the case of a relatively small number of layers, it is shown that the total electromagnetic power emitted is proportional to the square of the number of electrons in the macrobunch and inversely proportional to the number of microbunches; this power is also inversely proportional to the square of the internal radius of the structure for a cylindrical structure, and to the width of the vacuum core in a planar structure. Quantitative results are given for a higher number of dielectric layers, showing that in comparison to a structure bounded by metallic walls, the emitted power is significantly smaller due to propagation bands allowing electromagnetic energy to escape.

Particle-in-cell simulations of quasi-phase matched direct laser electron acceleration in density-modulated plasma waveguides

“…However, if the phase velocity of the optical field v p is greater than the electron velocity v e , the Woodward-Lawson theorem states that zero net energy gain is produced over an infinite acceleration distance. 6,7 For DLA methods operating in photonic bandgap (PBG) materials, 8,9 the working principle is to induce a resonant wave in the center channel (with a width on the scale of optical wavelength) which propagates with a phase velocity v p ≤ c to accelerate co-propagating electrons.…”

Bragg accelerator optimization

“…where κ, β gr , and Z int are dependent on the vacuum clearance [2] . In the case of a dielectric planar Bragg waveguide with a vacuum tunnel of 2D int along which a charged-line propagates, the wake coefficient associated with the decelerating field is…”

“…A planar Bragg waveguide [2] consists of dielectric layers surrounding a sub-wavelength vacuum region which is symmetrical relative to the central plane ( Figure 2). The clearance is a vacuum region of width 2D int , surrounding alternating periodic layers (ε 2 = 4, ε 3 = 2.1).…”

Analysis and Design of Optical Bragg Acceleration Structures

“…Subject to a design procedure [5,6], it is possible for the Bragg waveguides to support the TM mode with phase velocity c required for acceleration. Imposing the continuity of the transverse electric and magnetic fields at the boundaries between the layers, a matrix formulation is obtained, with which the location of the interfaces between the dielectric layers is determined.…”

“…Motivated by the low-loss Bragg dielectric planar mirrors used in high-power lasers, it is suggested to harness this concept in order to confine the laser-field in an optical acceleration structure [5,6]. Its essence is to form a hollow dielectric waveguide consisting of an almost perfect reflector made of a planar array of quarter-wavelength dielectric layers.…”